Method for evaluating the march of pressure in a combustion chamber

ABSTRACT

A method for evaluating the combustion chamber pressure in an internal combustion engine is described, in which the output signal of at least one cylinder pressure sensor and one crankshaft angle sensor is performed by the control unit of the engine. By analysis of the course of combustion chamber pressure over the crankshaft angle, characteristic pressure courses are obtained for certain valve control times. From these characteristic pressure courses, a conclusion can be drawn as to the valve control times “outlet opens”, “outlet closes”, “inlet opens”, and “inlet closes”, referred to the crankshaft angle.

BACKGROUND OF THE INVENTION

The invention relates to a method for evaluating the course of thecombustion chamber pressure and internal combustion engines.

It is known to ascertain the course of the combustion chamber pressuresin the cylinders of an internal combustion engine with the aid ofsuitable sensors, and from this course to detect operating states of theengine and obtain trigger signals for controlling the engine. Typically,each cylinder of the engine is assigned a combustion chamber pressuresensor. A crankshaft sensor is also used, which furnishes an outputsignal that is representative for the crankshaft position. The twosignals are evaluated jointly in the engine control unit. A camshaftsensor is no longer needed, since it is possible, especially afterstarting, to synchronize the crankshaft and camshaft position by linkingthe course of the combustion chamber pressure and the crankshaft sensorsignal. A method in which the course of combustion chamber pressure isevaluated as a function of the crankshaft position, for the sake ofcylinder detection and to generate signals required for ignition, isknown from published, unexamined German Patent Application DE-OS 44 05015. The cylinder detection and the detection of the crankshaftrevolution in which the engine is located in a combustion cycle isperformed in the known method by evaluating the pressure increase in acertain cylinder, for instance, and distinguishing between a pressureincrease in the compression stroke and a pressure increase in theensuing combustion. Since these values are different, the crankshaftrevolution in which the engine is located can be ascertained. From thisfinding, control signals for the engine can be generated.

In the known method, an evaluation of the course of combustion chamberpressure to detect the valve control times, that is, in order to detectwhether the outlet valve is opening or closing or whether the inletvalve is opening or closing, is not performed.

SUMMARY OF THE INVENTION

In keeping with these objects in accordance with the present invention amethod for evaluating a combustion chamber pressure in an internalcombustion engine includes performing measurements during normal engineoperations, and evaluating incident combustion chamber pressure coursesor events which depend on the combustion chamber pressure course andwhich characterize the valve control times.

The method of the invention has the advantage over the prior art thatprecise analysis of the course of combustion chamber pressure isperformed, so that the valve control times can be ascertained withreference to the crankshaft position. To that end, characteristic eventsare evaluated from which unambiguously determined valve control timescan be detected. For the valve control times of “outlet opens”, “outletcloses”, “inlet opens”, “inlet closes”, characteristic pressure coursesare obtained which according to the invention are advantageouslyextracted from the course of the combustion chamber pressure.

It is especially advantageous that various valve control times can beascertained by detecting the various associated characteristic events.Some valve control times can also be detected from a similar evaluationof the course of the combustion chamber pressure. A comparison withengine-typical characteristic variables stored in memory makes itpossible to determine valve control times for a specific engine.

Further processing of the combustion chamber pressure signal beforefurther evaluation, such as a differentiation or integration of thecourse of combustion chamber pressure, makes further ascertainments ofvalve control times advantageously possible. Taking additional engineoperating conditions into account, such as the incidence of knockingcombustion, and the ensuing additional signal processing, such asaveraging, advantageously makes it possible to ascertain valve controltimes even if difficult conditions or operating states of the engine areoccurring.

DESCRIPTION OF THE DRAWING

One exemplary embodiment of the invention is shown in the drawing FIGS.and will be described in further detail in the ensuing description.Specifically,

FIG. 1 shows a system, already known per se, for detecting the pressurecourse in the cylinders of an internal combustion engine.

In FIG. 1a, relevant parts of the internal combustion engine are shown.

FIG. 2 shows a characteristic course of combustion chamber pressure overthe crankshaft angle.

FIG. 3 is a flow chart of an evaluation method according to theinvention, and

FIGS. 4, 5 and 6 show various relationships among the combustion chamberpressure, combustion chamber volume, and crankshaft angle.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the most essential components of an apparatus forascertaining the combustion chamber pressure in each cylinder of aninternal combustion engine are shown. In each of the cylinders 10, 11,12 and 13 of a four-cylinder engine, respective cylinder pressuresensors 14, 15, 16 and 17 are disposed, which ascertain the pressurecourses P1, P2, P3 and P4. A crankshaft sensor 18 is also present, whichoutputs an output signal S1 that is characteristic for the crankshaftposition.

Both the output signals of the cylinder pressure sensors 14, 15, 16 and17 and the output signal of the crankshaft sensor 18 are delivered tothe engine control unit 19, which processes these signals. Via inputs20, further signals (such as a temperature T, load L and so forth) canbe supplied to the control unit and can be further processed in thecontrol unit as well.

The control unit 19 includes a multiplexer 21, by way of which theoutput signal of the cylinder pressure sensors is sent selectively to ananalog/digital converter 22. The switchover of the multiplexer 11 isdone as a function of crankshaft angle and is tripped by suitabletriggering actions on the part of the control unit 19. The actualevaluation of the signals is done in a microprocessor 23 of the controlunit 19; via an output unit 23 a, as a function of the variablesascertained, this microprocessor can output control signals S2 and S3,such as ignition or injection signals, to various components of theengine.

The signal processing takes place in the microprocessor 23 of thecontrol unit 19, and on the basis of this processing, a conclusion canbe drawn as to the valve control times, or the valve control times canbe ascertained.

FIG. 3 shows an evaluation flow chart, in which in step SCH 1 thepressure is calculated from the sensor signal. In step SCH 2, thecrankshaft angle is written in, so that in step SCH 3 the referencepressure P( ) is present. In step SCH 4, the pressure course isevaluated, optionally taking data stored in memory into account, and instep SCH 5, a conclusion as to the applicable valve control unit isdrawn.

By opening the inlet valve 24, the fuel-air mixture is supplied to thecylinder of an engine, for instance to cylinder 10 (FIG. 1a). In a knownmanner, the fuel is injected by the injection valve 25 before theinjection valve 24 into the intake tube 26, and ignited via the sparkplug 27 and via the spark plug 27. Via an outlet valve 28, the gasgenerated in the cylinder can be let out. The triggering of the inletand outlet valves is done in a known manner with the aid of the camshaftor camshafts, not shown. The camshaft or camshafts are driven in a knownmanner by the crankshaft. The location of the camshaft or camshaftsrelative to the crankshaft can be varied by the control unit 19 as afunction of rpm, by means of suitable trigger signals S3. By thedetection according to the invention of the valve control times as afunction of the crankshaft angle, the association between the camshaftposition and the crankshaft position can be determined.

In FIG. 2, the course of the combustion chamber pressure P1 of thecylinder 10 is plotted over the crankshaft angle. The cylinder pressureattains two maximum values, which are one cycle or 720 KW apart. Themaximum combustion chamber pressure in the range in which a combustionoccurs is higher than in the range in which only a compression occurs.In the example of FIG. 2, a combustion takes place in the phase Ve. Inthe phase Ko only a compression occurs.

The combustion chamber pressure course schematically shown in FIG. 2 isevaluated according to the invention by various criteria, in order fromthem to draw conclusions as to events that are characteristic for thecamshaft position relative to the crankshaft position and thus for thevalve seat control times. One such event can for instance be thecrankshaft position at which the inlet valve closes. Other valve controltimes are the control times designated as “outlet opens”, “inlet opens”,and “outlet closes”. For each valve control time, there arecharacteristic or definitive features in the pressure course, theevaluation of which features will be described in further detail below.

To detect the valve control time “outlet opens”, the expansion line ofthe combustion chamber pressure course can be evaluated. As long as theoutlet valve is closed, the events occurring in the cylinder involve athermodynamically closed system, so that the events can be calculated inaccordance with thermodynamic principles. As the volume increases, apressure decrease occurs, which is established similarly to a polytropicexpansion. It is characteristic of this that the amount of the pressuregradient decreases with increasing volume. If the outlet valve isopened, then dictated by the pressure that is elevated relative to theenvironment, gas flows out of the cylinder. As a result, the amount ofthe pressure gradient increases. The evaluation of the pressure gradientfor the outlet opening that has occurred can thus utilized as adefinitive or characteristic behavior of the pressure course. If thepressure gradient has a behavior which is distinguished by a lesseningdecrease and a sudden increase in the amount of the pressure gradientthen it can be concluded that the outlet has opened. Mathematically, theevaluation can be done by checking for instance for a change of sign inthe second derivation of the pressure in accordance with the volume. Ifsuch a change of sign occurs in the second derivation of the pressure inaccordance with the crankshaft angle, then it can be concluded that anoutlet opening has taken place. In FIG. 4, which shows the relationshipbetween the pressure P and the volume V between top dead center OT andbottom dead center UT, the point A1 would characterize the outletopening that has occurred. At this point, it is true that the secondderivation of the pressure in accordance with the volume d²P/dv has achange of sign. This is also true for the relationship d²P/d².

To detect the valve control time “inlet closes”, the volume or thecrankshaft angle at which the compression curve passes through a known,fixed level is detected. In the simplest case, this comparison level isobtained from the pressure course during expulsion. The location of theintersection A2 between the compression pressure course and the pressurecourse during the expulsion in the crankshaft angle pattern or thecourse of volume can be learned from FIG. 5. It is admittedly not adirect measure of the valve control time “inlet closes”, but it doesshift upon a change in the closure of the inlet valve. Thus a desiredvalue for the location of point A2 can be applied in engine-dependentfashion as a function of the load and rpm. For a diagnosis, thedeviation of the actual value for the point A2 from the desired value isthen used. The recording of the engine-specific data can be done beforethe engine is put into operation, for instance on a test bench. The dataobtained are then stored in memories, for instance of the control unit,which can access these data at any time.

The evaluation of the course of combustion chamber pressure is notlimited to only the pressure-volume relationship; an evaluation on thebasis of the pressure and crankshaft angle relationship is alsopossible. By evaluating the location of points A3 and A4 in FIG. 6,corresponding conclusions can be drawn. Also plotted in FIG. 6 is thecombustion chamber pressure P over the crankshaft angle. In addition,the load change top dead center points LWOT, an ignition top dead centerpoint ZOT, bottom dead center points UT, and angles α3, β3, α4, β4 areplotted; the angle α3 and α4 respectively defines the distance betweenbottom dead center UT and the respective point A3 and A4; the angle β3defines the distance between A3 and ZOT; and the angle β4 defines thedistance between A4 and LWOT. If the pressure at point A3 is equal tothe pressure at point A4, than for the angles the applicable equationsare α3=α4 and β3=β4.

If it is not possible to evaluate the course of combustion chamberpressure during the expulsion of the combustion gases located in thecylinder, for instance if because of the high combustion temperature,from transient drifting caused by thermal shock, the combustion chamberpressure sensor furnishes only imprecise signals, then the evaluation ofthe course of combustion chamber pressure can also be performed as asubstitute by comparison with the ambient pressure. For instance, todetect the valve control time “inlet valve closes inlet valve”, thevolume or the crankshaft angle at which the compression pressure isequal to the ambient pressure can be detected. In that case, the pointA3 is defined as the intersection of the compression pressure course andthe ambient pressure. Then, however, a zero level correction of thepressure course will be necessary, which increases the effort andexpense of calculation and under some circumstances can lead toincorrect measurements.

If measurement values for the ambient pressure and the compressioncurve, which is the case in supercharged engines, for instance, than anevaluation of the combustion chamber pressure course can also be done onthe basis of a fixed pressure value. In that case, however, specialdiagnostic strategies that prevent misdiagnosis from a strong change inthe ambient pressure, for instance when driving at relatively highaltitudes, are necessary. If the control unit detects this kind of highaltitude travel, for instance in conjunction with other evaluations forregulating the engine, then a detection of valve control times can besuppressed at least intermittently.

If valve control times change, for instance because of a correspondingchange in the camshaft positions, once again this leads to a change inthe combustion chamber pressure course during the compression phase, thecombustion phase, and the expansion phase. From a change in the camshaftposition, the valve control times are for instance changed in such a waythat the residual gas content in the cylinder charge varies in acharacteristic way. A relatively high residual gas content, which can becaused for instance by late closure of the outlet valve or early openingof the inlet valve, in each case relative to the crankshaft angle,increases both the absolute pressure and the pressure gradient duringthe compression phase, assuming that the same quantity of fresh air isdelivered. If the same instant of ignition is assumed, the combustionwill begin late, with the attendant effects on the characteristic valuesthat describe the combustion and the expansion. If variousengine-specific characteristic values or performance graphs are storedin memories of the control unit, than these characteristic values orperformance graphs can be accessed at any time. A comparison with themeasured cylinder pressure course, with knowledge of theengine-specifically present relationships, for instance also includingascertained mathematical relationships, yields a conclusion as to whichof the valve control times is present. During engine operation,characteristic values can be adapted. From the adapted characteristicvalues, once again a conclusion as to the current valve control timescan be drawn. A further evaluation option for the course of combustionpressure can also be obtained from the deviation from cycle to cycle inthe variables characterizing combustion, in externally ignited engines,with an increasing residual gas content. This affords the opportunity ofmaking a conclusion about the valve control times from the deviation inthe characteristic values via engine-specifically ascertainedperformance graphs or characteristic curves, engine-specificallyascertained mathematical relationships, or characteristic values adaptedduring engine operation.

A combination of the aforementioned evaluation options can be made atany time. It is also possible, both in evaluating the pressure gradientsand in evaluating the maximum pressure, the location of the maximumpressure, and in general in the evaluation of single pressure courses,first to perform averaging, for instance over multiple engine cycles,and then to examine the average values of the combustion chamberpressure course for variables that characterize certain valve controltimes. Once again, engine-specifically ascertained relationships, storedin memory as a performance graph or characteristic curve, ormathematical relationships should be taken into account. To detect atleast one of the valve control times “outlet opens”, “outlet closes”,“inlet closes”, “inlet opens”, a defined combustion chamber pressureintegral or a differential combustion chamber pressure integral can alsoinitially be formed; the integration limits should be selected in asuitable way and in particular designed such that valve controltime-typical phases are combined.

A further option for detecting the valve control times is to derivecharacteristic variables for certain valve control times from theoccurrence of oscillations in the combustion chamber pressure course asa consequence of knocking combustion or from the necessity of counterprovisions to avoid knocking combustion, which provisions are in turntaken on the basis of pressure oscillations in the course of thecombustion chamber pressure. Once again, an additional averaging can beperformed.

The invention can be used in engines with an arbitrary number ofcylinders; the number of cylinder pressure sensors is for instance equalto the number of cylinders or to half the number of cylinders. In asimplified version, at least one sensor can be employed. As the sensors,knocking sensors can also be used, or arbitrary combustion sequencesensors, from whose output signal characteristic features for valvecontrol times can be obtained.

What is claimed is:
 1. A method for evaluating a combustion chamberpressure in an internal combustion engine having at least one cylinderpressure sensor, which measures a cylinder pressure, and one crankshaftangle sensor, which outputs a signal representative of a crankshaftposition, and one evaluation device, including at least onemicroprocessor, to which signals of the sensors are supplied, in whichthe microprocessor, from a course of the combustion chamber pressure asa function of the crankshaft angle position, concludes that at least oneof valve control times “outlet opens”, “outlet closes”, “inlet opens”,“inlet closes” exists with respect to the crankshaft angle position,characterized in that measurements are performed during normal engineoperation, and incident combustion chamber pressure courses or eventswhich depend on the combustion chamber pressure course and whichcharacterize the valve control times are evaluated.
 2. The method ofclaim 1, characterized in that it is concluded that the valve controltime “outlet opens” exists, if an expansion line of the course ofcombustion chamber pressure is varying in such a way that a change inthe pressure gradient changes its sign with increasing volume or with anincreasing crankshaft angle.
 3. The method of claim 1, characterized inthat to detect “inlet closes”, a volume or the crankshaft angle at whicha compression pressure is equal to a pressure that prevailed during anexpulsion at a same distance from top dead center is detected.
 4. Themethod of claim 1 or 2, characterized in that to ascertain the valvecontrol time “inlet closes”, a volume or the crankshaft angle at whichthe compression pressure is equal to an ambient pressure is detected. 5.The method of claim 1 or 2, characterized in that to detect the valvecontrol time “inlet closes”, a volume or the crankshaft angle at whichthe compression pressure is equal to a predeterminable fixed pressure isdetected.
 6. The method of claim 1, characterized in that to determinethe valve control times “outlet closes”, “inlet closes” or “inletopens”, an absolute pressure level during a compression before an onsetof combustion is evaluated, and either from a single pressure course orfrom a pressure course averaged over multiple cycles, by comparison withengine-specifically ascertained data stored in memory in form of aperformance graph or characteristic curve or engine-specificallyascertained mathematical relationships or adapted conversion factors, aconclusion as to the valve control times is drawn.
 7. The method ofclaim 1, characterized in that to detect at least one valve controltime, a combustion pressure gradient during a compression before anonset of combustion or a polytropic exponent calculated from it isevaluated, and from a single pressure course or from a pressure courseaveraged over multiple cycles, via engine-specifically ascertainedconversions stored in memory in form of a performance graph orcharacteristic curve or engine-specifically ascertained mathematicalrelationships or adapted conversion factors, a conclusion as to thevalve control time is drawn.
 8. The method of claim 1, characterized inthat from an absolute pressure level or from a pressure gradient duringan expansion before opening of the outlet valve or from a polytropicexponent calculated from pressure gradients, either from a singlepressure course or from a pressure course averaged over multiple cycles,via engine-specifically ascertained conversions stored in memory in formof a performance graph or characteristic curve, engine-specificallyascertained mathematical relationships or adapted conversion factors, aconclusion is drawn as to the valve control times “outlet closes”,“inlet opens” or “inlet closes”.
 9. The method of claim 1, characterizedin that from a location of a maximum pressure increase, either from asingle pressure course or from a pressure course averaged over multiplecycles, via engine-specifically ascertained conversions stored in memoryin form of a performance graph or characteristic curve,engine-specifically ascertained mathematical relationships or adaptedconversion factors, a conclusion is drawn as to the valve control times“outlet closes”, “inlet opens” or “inlet closes”.
 10. The method ofclaim 1, characterized in that to detect the valve control times, atleast one of the following variables is employed: deviation of alocation of the maximum pressure increase over multiple cycles, amaximum incident pressure gradient from a single pressure course or froma pressure course averaged over multiple cycles; deviation of themaximum incident pressure gradient over multiple cycles; location of themaximum pressure from a single pressure course or from a pressure courseaveraged over multiple cycles; deviation of the location of the maximumpressure over multiple cycles; level of the maximum pressure from asingle pressure course or from a pressure course averaged over multiplecycles; deviation of the location of the maximum pressure over multiplecycles; wherein additionally, engine-specifically ascertainedconversions stored in memory in form of a performance graph orcharacteristic curve, engine-specifically ascertained mathematicalrelationships or adapted conversion factors are also taken into accountto determine the valve control times.
 11. The method of claim 1,characterized in that a conclusion as to the valve control times isdrawn from one of the following variables: deviation of a location ofcertain portions of the energy conversion over multiple cycles; locationof a maximum energy conversion from a single pressure course or from apressure course averaged over multiple cycles; deviation of the locationof the maximum energy conversion over multiple cycles; maximum gradientof the energy conversion from a single pressure course or from apressure course averaged over multiple cycles; and taking into accountengine-specifically ascertained conversions stored in memory in form ofa performance graph or characteristic curve, engine-specificallyascertained mathematical relationships or adapted conversion factors.12. The method of claim 1, characterized in that one of the followingvariables is evaluated: indicated work from a single pressure course orfrom a pressure course averaged over multiple cycles; deviation in theindicated work over multiple cycles; indicated high-pressure work from asingle pressure course or from a pressure course averaged over multiplecycles; deviation of the indicated high-pressure work over multiplecycles; indicated low-pressure work from a single pressure course orfrom a pressure course averaged over multiple cycles; deviation in theindicated low-pressure work over multiple cycles, and a conclusion as tothe valve control times is drawn via engine-specifically ascertainedconversions stored in memory in form of a performance graph orcharacteristic curve, engine-specifically ascertained mathematicalrelationships or adapted conversion factors.
 13. The method of claim 1,characterized in that the combustion chamber pressure is integrated overa predeterminable range, or that a differential combustion chamberpressure is integrated over a predeterminable range, and either anintegral from a single pressure course or from a pressure courseaveraged over multiple cycles is formed, and a conclusion as to thevalve control times is drawn via engine-specifically ascertainedconversions stored in memory in form of a performance graph orcharacteristic curve, engine-specifically ascertained mathematicalrelationships or adapted conversion factors.
 14. The method of claim 13,characterized in that from the deviation in the integral or integralsover multiple cycles, a conclusion as to the valve control times isdrawn.
 15. The method of claim 1, characterized in that from anoccurrence of oscillations in the course of the combustion chamberpressure caused by knocking combustion or from a necessity ofcounterprovisions to avoid knocking combustion, which in turn are takenon a basis of pressure oscillations in the course of the combustionchamber pressure, a conclusion as to the valve control times is drawneither from a single pressure course or from a pressure course averagedover multiple cycles, via engine-specifically ascertained conversionsstored in memory in form of a performance graph or characteristic curve,engine-specifically ascertained mathematical relationships or adaptedconversion factors.